Sensing of Abiotic Stress and Ionic Stress Responses in Plants

16 Pages·2017·0.75 MB·English

International Journal o f
Molecular Sciences
Review
Sensing of Abiotic Stress and Ionic Stress Responses
in Plants
YuZhang† ,YangLv†,NoushinJahan,GuangChen ,DeyongRenandLongbiaoGuo*
StateKeyLabforRiceBiology,ChinaNationalRiceResearchInstitute,Hangzhou310006,China;
[email protected](Y.Z.);[email protected](Y.L.);[email protected](N.J.);
[email protected](G.C.);[email protected](D.R.)
* Correspondence:[email protected];Tel.:[phone]
† Theseauthorscontributedequallytothiswork.
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:29September2018;Accepted:23October2018;Published:24October2018
Abstract:Plantsneedtocopewithcomplexenvironmentsthroughouttheirlifecycle. Abioticstresses,
includingdrought,cold,saltandheat,cancauseareductioninplantgrowthandlossofcropyield.
Plants sensing stress signals and adapting to adverse environments are fundamental biological
problems. Wereviewthestresssensorsinstresssensingandtheresponses,andthendiscussionic
stresssignalingandtheresponses. Duringionicstress, thecalcineurinB-likeproteins(CBL)and
CBL-interacting protein kinases (CBL−CIPK) complex is identified as a primary element of the
calcium sensor for perceiving environmental signals. The CBL−CIPK system shows specificity
and variety in its response to different stresses. Obtaining a deeper understanding of stress
signalingandtheresponseswillmitigateorsolvecropyieldcrisesinextremeenvironmentswith
fast-growingpopulations.
Keywords: abioticstress;ionicstress;sensing;response;CBL−CIPK
1. Introduction
Plants cannot escape adverse environmental conditions [1]. Due to the constantly changing
environment,abioticstressisthemainfactoraffectingcropgrowthandproductivity,asplantsare
sessileorganisms. Abioticstressincludessoilsalinity,droughtandextremetemperatureconditions
(cold or hot) [2]. Anthropogenic activities directly affect global climate change. Identifying the
mechanismsthroughwhichplantscounteractabioticstressandmaintaintheirgrowthandsurvival
holdssignificanceforplantsincopingwithglobalclimatechange[3–5].
There are inherent physical, morphological and molecular limitations to the plant’s ability to
respond to different abiotic stresses [6]. Although plants differ physically, morphologically and
molecularlybetweenwild-typeandmoderncultivarsintermsoftheirstresssensingability,theirstress
sensingmechanismsarestillpoorlyunderstood.Toovercomethislimitation,aninitialstepindecoding
theprocessinvolvesstresssensorsignalsthatareproducedbytheplantcell. Studiesoncalcineurin
B-likeproteins(CBLs)andtheirtargets,CBL-interactingproteinkinases(CIPKs),generallyknownas
theCBL−CIPKnetwork,provideusefulmodelsforunderstandingthemechanismsofstresssignal
transductioninplants[2].
Coldstresscanrigidifytheplasmamembraneandrearrangethecytoskeletonsocalciumcan
pass through calcium channels into the cytoplasm [7]. The calcium signal is then decoded by a
calcium-bindingprotein,whichleadstodownstreamactionsthatdeliverthecoldsignal[7]. COLD1,
a quantitative trait locus gene identified in rice that encodes a regulator of G-protein signaling
localizedontheplasmamembraneandendoplasmicreticulum,wasfoundtocausetheG-proteinα
subunittoinitiatetheguanosinetriphosphatase(GTPase)activityofregulatorofG-proteinactivate
Int.J.Mol.Sci.2018,19,3298;doi:10.3390/ijms19113298 www.mdpi.com/journal/ijms
Int.J.Mol.Sci.2018,19,3298 2of16
1(RGA1)[8]. COLD1/RGA1representsapotentialcalciumpermeablechannel,asalackofCOLD1
affectscalcium(Ca2+)signalingwithcoldstressinrice[7,8].Phytochromes(Phy),plantphotoreceptors,
regulate photomorphogenesis, and PhyB, the primary photoreceptor, controls plant growth in
Arabidopsis,bothincoldandhightemperature(HT)stress[7,9,10]. UndernormalandHTconditions,
phytochrome interacting factor 4 (PIF4), a basic helix-loop-helix transcription factor, forms part
of the central regulatory hub mediating the diurnal growth of plants [9]. It was proposed that
thephyB−PIF4signalingmoduleplaysanimportantroleinbalancingplantgrowthanddefenses
duringtheresponsetoHTstress[11]. PIF4canintegratebrassinosteroid(BR)andgibberellin(GA)
signaling. Evidence has shown that PIF4 directly binds to the promoters of DWARF4 (DWF4)
and BRASSINOSTEROID-6-OXIDASE2 (BR6ox2), and then promotes the expression of these BR
biosynthesisgenes[12].
Soilsalinizationisaworldwideproblemthatseriouslythreatensagricultureasitrestrictsthe
growthandyieldofcrops[13]. Saltstressincludestwotypes: osmoticstressandionicstress. Riceis
knowntobeasalt-sensitivecropandtheannotationsofthericegenomesequenceenablethestudy
of the functional genomics of the salt stress response [14]. Oryza sativa Salt tolerance activation 2
(OsSta2) was identified and studied in rice. Under salt stress, OsSta2-Ox (overexpression) plants
weremoretoleranttoosmoticstressandcouldmaintainamuchhealthiergrowthpatternthanwild
type (WT) seedlings in response to mannitol treatment, which indicates that OsSta2 may respond
to salt and drought stress [15]. Studies have found that some RNA functions are also involved in
cropsalttolerancemechanisms[16–19]. WithRNA-seqandsRNA-seq,therewere2574mRNAsand
76miRNAs,respectively,thatweredifferentiallyexpressedincitrusrootundersaltanddehydration
treatments[17].Inmaize,eightnovelmiRNAsandtheirtargets,inresponsetosalinitystress,havebeen
identified. Atotalof37potentialnewmiRNAswereselectedbasedonthesamecriteriainresponseto
saltstress[16]. TheinteractionbetweenmiRNAsandtheirtargetsmayalsoplayimportantrolesin
responsetosalinity.
MultipleMitogen-activatedproteinkinases(MAPKs)havebeenobserved,whichcanrespondto
abioticstimulisuchassalt,drought,cold,heatandwounding,aswellastogrowthanddevelopmental
signals[20]. Abscisicacid(ABA)hasbeenshowntomodulatethetranscription,proteinaccumulation
andkinaseactivityofseveralMAPKsignalingcomponents[20]. TheABA-signalingpathwayiscentral
to drought and salt stress responses in plants [2,21]. The interactions of CIPK1, CBL1 and CBL9
formdifferentcomplexesthatseemtoworkindistinctprocesses,includingdrought-responsivegene
expressionandABAresponses.
Therefore,understandingthephysiologicalandmolecularaspectsofplantstresssensingfunctions
andimprovingplantstressresistancearecriticalforagriculturalgrowthandproductivity.
2. SensingofOrganellarStress
Plantcellsarecapableofsensingvariousenvironmentalsignalsinresponsetodifferentabiotic
stress conditions. The plant then demonstrates variations in gene expression, metabolism and
physiology [2]. Research has revealed abiotic stress signaling perception in plants. Sensors that
recognizetheinitialstresssignalinitiatecascadesthattransmitthesignaltodifferentorganellesand
activatetranscriptionfactors(TFs)thatinducetheexpressionofaparticularsetofgenes. Asingle
sensorcanregulatethebranchesofthesignalcascade,whichistriggeredbyoneaspectofthestress
condition(Figure1).
Int.J.Mol.Sci.2018,19,3298 3of16
Int. J. Mol. Sci. 2018, 19, x FOR PEER REVIEW 3 of 15

Figure1.Stresssensingandsignaling;amodeloftheresponsetoabioticstressesindifferentorganelles.
Figure 1. Stress sensing and signaling; a model of the response to abiotic stresses in different
The signal generated by organelles can cause gene expression and cellular activities, which can
organelles. The signal generated by organelles can cause gene expression and cellular activities, which
restorecellularhomeostasisunderabioticstress.Arrowsindicateactivationandsignaltransduction,
can restore cellular homeostasis under abiotic stress. Arrows indicate activation and signal
anddashedlinesindicateresponse.
transduction, and dashed lines indicate response.
Stress-sensing frequently occurs on the cell surface or at the cell membrane [22]. From there,
Stress-sensing frequently occurs on the cell surface or at the cell membrane [22]. From there, the
thesignalcanberelayedtodifferentorganelles. Proteinfoldingintheendoplasmicreticulum(ER),
signal can be relayed to different organelles. Protein folding in the endoplasmic reticulum (ER),
referredtoasERstress,iswidelyrecognizedasanimportantcellularresponsetostress,whichcan
referred to as ER stress, is widely recognized as an important cellular response to stress, which can
be caused by both biotic and abiotic stress [2,23]. ER membrane-associated transcription factors,
be caused by both biotic and abiotic stress [2,23]. ER membrane-associated transcription factors, basic
basic leucine zipper (bZIP) proteins, are important for restoring ER homeostasis [24]. The bZIP
leucine zipper (bZIP) proteins, are important for restoring ER homeostasis [24]. The bZIP
transcriptionfactors(TFs)constituteoneofthelargestfamiliesinplantswithnearly15,498genesfrom
transcription factors (TFs) constitute one of the largest families in plants with nearly 15,498 genes
166species[25].
from 166 species [25].
Basedontheplanttranscriptionfactordatabase,differentkindsandnumbersofTFsarefound
Based on the plant transcription factor database, different kinds and numbers of TFs are found
indifferentplantspecies,whichareidentifiedaslociandclassifiedintodifferentfamilies(Table1).
in different plant species, which are identified as loci and classified into different families (Table 1).
For instance, the gene annotation from TAIR10 is used to identify transcription factors (TFs) of
For instance, the gene annotation from TAIR10 is used to identify transcription factors (TFs) of
Arabidopsisthaliana. Accordingtothefamilyassignmentrules,2296TFs(1717loci)havebeenidentified
Arabidopsis thaliana. According to the family assignment rules, 2296 TFs (1717 loci) have been
andclassifiedinto58families[25].
identified and classified into 58 families [25].
Table1.Differenttranscriptionfactors(TFs)andlociindifferentplantspecies.
Table 1. Different transcription factors (TFs) and loci in different plant species.
TaxonomicGroup LatinName Species TFs Loci Families
Taxonomic Group Latin Name Species TFs Loci Families
Oryzasativasubsp.japonica 2408 1862 56
Oryza sativa subsp. japonica Rice 2408 1862 56
Oryzasativasubsp.indica Rice 1891 1891 56
Monocots OryzaS soartgihvuam subibcoslpo.r indica Sorghum 12869541 189118 59 56 56
Monocots TSroitrigcuhmumae bstiicvoulomr WSohregahtum 23665064 185936 06 56 56
Zeamays Maize 3308 2289 56
Triticum aestivum Wheat 3606 3606 56
ArabiZdoepas misathyasl iana ArabMidoapizsies 32320968 228917 17 56 58
Nicotianatabacum Tobacco 5176 3625 57
Eudicots Arabidopsis thaliana Arabidopsis 2296 1717 58
Solanumlycopersicum Tomato 1845 1845 58
Nicotiana tabacum Tobacco 5176 3625 57
Solanumtuberosum Potato 2405 1736 56
Eudicots
Solanum lycopersicum Tomato 1845 1845 58
Solanum tuberosum Potato 2405 1736 56
Theproductionofreactiveoxygenspecies(ROS),includingsuperoxideanion,hydrogenperoxide
(H O ),hydroxylradicalsandsingletoxygen,mainlyaffectschloroplasts[26,27]. Asthemajorcellular
2 2
The production of reactive oxygen species (ROS), including superoxide anion, hydrogen
componentforphotosynthesis,chloroplastsarehighlyexposedtoROSdamagesincephotosynthesisis
peroxide (H2O2), hydroxyl radicals and singlet oxygen, mainly affects chloroplasts [26,27]. As the
asourceofROS,especiallyH O [28]. Likechloroplasts,mitochondriaandperoxisomescangenerate
2 2
major cellular component for photosynthesis, chloroplasts are highly exposed to ROS damage since
retrogradesignals.TheyproduceROSandmanymetabolitesthatmayaffectcalciumsignaling,someof
photosynthesis is a source of ROS, especially H2O2 [28]. Like chloroplasts, mitochondria and
whichmayserveasretrogradesignals[29–31].
peroxisomes can generate retrograde signals. They produce ROS and many metabolites that may
The primary cell wall of the plant is composed of cellulose fibrils, which are connected by
affect calcium signaling, some of which may serve as retrograde signals [29–31].
hemicellulosetethersandembeddedinapectingel[22]. Thepectinsareoftenmodifiedwhenexposed
The primary cell wall of the plant is composed of cellulose fibrils, which are connected by
to drought stress. By comparing the cell wall changes of two different drought-resistant wheat
hemicellulose tethers and embedded in a pectin gel [22]. The pectins are often modified when
varietiesunderstress,anincreaseinthepectinpolymersrhamnogalacturonanIandII(RGIandRGII)
exposed to drought stress. By comparing the cell wall changes of two different drought-resistant
wheat varieties under stress, an increase in the pectin polymers rhamnogalacturonan I and II (RGI
Int.J.Mol.Sci.2018,19,3298 4of16
side chains was found to probably be due to the hydrogel formation of pectin, which limited the
damage to the cells [32]. The cell wall also contains phenolics, enzymes, proteins, and Ca2+. Salt,
drought,andotherosmoticstresstreatmentscanleadtotheaccumulationofROSinthecellwall[22].
The dirigent (DIR) proteins in plants modulate the cell wall metabolism during abiotic and biotic
stress[33]. Someproteinsinthecellulosesynthasecomplexhelpconnectthecomplextomicrotubules
forplantgrowthduringsaltstress[34]. Membersofdifferentreceptor-likekinasesperceivethestress,
whichincludesalargefamilyofintegralplasmamembraneproteins. Thesekinases,uponperceiving
stress, can transmit their signal into the cell [35]. Constitutive expression of receptor-like protein
kinase1(RPK1)cancauseupregulationofanumberofstress-inducedgenesandenhanceabioticstress
toleranceinArabidopsis[36].
As well, the changes in organellar gene expression (OGE) in response to developmental and
environmentalchanges,andperturbationsofOGEhomeostasis,regularlyresultintheactivationof
toleranceresponses[37].
3. IonicStressSignaling
Highsalinitystresscausesanionhomeostasisimbalanceinsidetheplantcell. Plantcellsproduce
anionicstresssignal. Thesalinitystresssignalisthenperceivedbyareceptororsaltsensorpresent
at the plasma membrane of the cell, which is generally regulated by the coordinated action of
varioussaltoverlysensitive(SOS)pathwaysandionpumps,anditsdownstreaminteractingpartners,
whichultimatelyresultsintheeffluxofexcessions
3.1. Sodium
Detrimentalionicstress,especiallysaltstress,includingchloride(Cl−),carbonate(CO 2−)and
3
sulfate(SO 2−)saltsofsodium(Na+),magnesium(Mg2+)orcalcium(Ca2+),isharmfultoplantsand
4
occursindifferentgeographicalareasaroundtheworld[38]. Na+ hasadeleteriouseffectwhenit
accumulatesincellsandcertaintissues,directlyinterferingwithcellularfunctions. Plantsoffsetthe
initialosmoticcomponentsofsaltstressbyadjustingtheosmoticgradient,althoughtheaccumulation
ofNa+canleadtotoxiceffectsinthelong-term[39]. Na+affectsthehydrationshellofothermolecules,
causesdamagetothecellwall,disturbstheK+/Na+ratioofcellsbyseveralmechanismsandimpairs
plant physiology [40–42]. Plants use a calcium-dependent protein kinase pathway, known as the
SOSpathway,forsaltstresssignalingandNa+ tolerance[21]. Untilrecently,asensororreceptorof
Na+ hasnotbeenidentifiedinplants[43,44]. Inplants,theCa2+ sensorrelayproteinsdonothave
anykinaseactivity(Figure2);theyinteractwithsensorresponderproteinstomodulatedownstream
reactionsafterbindingtoCa2+. ThenameCBLhasbeenwidelyusedtorefertoafamilyofEF-hand
calcium-bindingproteins[45].
IntheplantSOSpathway,SOS3(CBL4),anEF-handcalcium-bindingprotein,sensesthecytosolic
calciumsignal. SOS3subsequentlyinteractswith,andactivates,theCBL-interactingproteinkinase
SOS2(CIPK24)[46,47]. DespitetheabilityofSOS3tobindCa2+,theinvitroformationandactivation
ofSOS3-SOS2complexeswasnotenhancedbyincreasingCa2+concentrations[48]. TheactivatedSOS2
phosphorylatesandactivatesSOS1,aNa+/H+ antiporterattheplasmamembrane[2,21]. Dysfunction
inanyoftheSOSgenesincreasesthesensitivityofthemutantplantstosaltstress. Thesos1mutants
appearedtobethemostsalt-sensitive,followedbysos2mutantsexhibitingintermediatesaltsensitivity,
withsos3mutantsbeingtheleastsaltsensitive[49].
Int.J.Mol.Sci.2018,19,3298 5of16
Int. J. Mol. Sci. 2018, 19, x FOR PEER REVIEW 5 of 15

FFiigguurree 22.. TThhee aabbiioottiicc ssttrreessss ooff hhiigghh ssooddiiuumm,, llooww ppoottaassssiiuumm aanndd hhiigghh ppoottaassssiiuumm ssttrreessss.. DDiiffffeerreenntt ccoolloorrss
iinnddiiccaattee ddiiffffeerreenntt ppaatthhwwaayyss.. AAKKTT11:: AArraabbiiddooppssiiss KK++ ttrraannssppoorrtteerr 11,, AAKKTT22:: AArraabbiiddooppssiiss KK++ ttrraannssppoorrtteerr 22,,
HHKKTT11:: hhiigghh--aaffffiinniittyy ppoottaassssiiuumm ttrraannssppoorrtteerr 11,, SSOOSS11:: ssaalltt oovveerrllyy sseennssiittiivvee.. AArrrroowwss iinnddiiccaattee aaccttiivvaattiioonn,,
aanndd bbaarrss iinnddiiccaattee iinnhhiibbiittiioonn..
SSOOSS11 aanndd hhiigghh--aaffffiinniittyy ppoottaassssiiuumm ttrraannssppoorrtteerr 11 ((HHKKTT11)) hhaavvee aannttaaggoonniissttiicc ffuunnccttiioonnss.. TThhee HHKKTT
pprrootteeiinn ffaammiillyy hhaass bbeeeenn sshhoowwnn ttoo bbee ccrriittiiccaall ffoorr ssaalliinniittyy ttoolleerraannccee iinn ppllaannttss [[5500]].. HHKKTT11 iinn AArraabbiiddooppssiiss
iiss aa NNaa++ iimmppoorrtteerr,, eexxpprreesssseedd iinn ppaarreenncchhyymmaa cceellllss aanndd ootthheerr cceellllss iinn tthhee vvaassccuullaarr ssyysstteemm tthhrroouugghhoouutt
tthhee ppllaanntt [[5511,,5522]].. IInn tthhee rroooottss,, HHKKTT11 uunnllooaaddss NNaa++ ffrroomm tthhee xxyylleemm iinn oorrddeerr ttoo lliimmiitt tthhee aammoouunntt ooff NNaa++..
IInn tthhee lleeaavveess, ,HHKKTT11 wwaas sththoouugghht ttot obeb aebalbe lteo tlooaloda NdaN+ ian+toin tthoe tphhelopehmlo eamnda rnedcyrcelcey ict lbeaictkb taoc tkhteo rothoet
[r5o3o]t. [A53 m].uAtamtiuonta otifo HnoKfTH1 KcaTu1sceadu osebdvioobuvsi oseunsssietinvsiittyiv tioty satolt ssatlrtesstsr,e bsus,t binuhtiinbihtiebdit tehdet shaelts aseltnsseintisviittiyv iotyf
SoOfSSO mSumtaunttasn itns tinhet hceulctuulrteu rmeemdieudmiu m[53[]5.3 ].
UUnnddeerr ssaalltt ssttrreessss,, OOssHHKKTT11;;11,, aa NNaa++ ttrraannssppoorrtteerr eexxpprreesssseedd iinn tthhee vvaassccuullaarr ttiissssuuee ooff rriiccee sshhoooottss,,
eennhhaanncceess NNaa++ iioonn eexxcclluussiioonn ffrroomm tthhee cceellll [[5544,,5555]].. MMoossttllyy iinn mmoonnooccoott ppllaannttss,, HHKKTT11 wwaass ffoouunndd ttoo
ffuunnccttiioonn iinn ththee vvasacsuculalra rtistsisuseu oef orfoorotso atsnda nsdhosohtos,o atns,da rnedgurleagtue lNatae+ iNona+ mioovnemmoenvte mfroemnt rforootm tor sohoototto.
Isnh osoaltt.-tIonlesaraltn-tt ollaenrdanratcleasn, durnalciekse, OunsHlikKeTO1;s1H, tKhTe1 O;1s,HthKeTO1;s4H gKeTne1 ;4mgoerne eefmfiocireenetlfyfi ccioennttrlyibcuotenst rtiob uNteas+
etoxcNluas+ionex fcrloumsi olenaff brolamdelse aaft lbelaafd sehseaatthsle, acofmshpeaartehds ,wciothm tphae rjeadponwicitah ritchee spjaepcoiensi c[a56r]i.c Ien sjpapeocnieicsa [r5ic6e].
cInuljtaivpoanrsic, aOriscHeKcTu1lt;i4v adrso,wOns-HreKgTu1la;4teds owNna+- reigoun lactoesncNena+traiotinonc oninc enletaravteiso nbiyn lreeagvuelsatbinygr egNual+a tiionng
mNao+veimonenmtso fvreomme snttesmf rtoom leastveems atto thleea vveegseatatttihvee svteaggeet,a btiuvte hsatsa ngeo, ibmuptahcta sonn oloiwmepriancgt tohne lhoiwghe rNinag+
itohne choignhceNntar+atiioonn cdoirneccetnlytr fartoiomn ldeairveecst l[y57fr,5o8m]. Ale asvtuesdy[5 s7h,5o8w].eAd tshtautd myushtaonwtse wditthha atnm ouvtearnetxspwreisthsioann
oofv OerseHxpKrTe1s;s4io anreo mfOorseH sKeTn1s;i4tivaree tom soarlet tsheannsi WtivTe ptolasnatlst [t5h9a]n. OWsTHKplTa1n;t5s, [k5n9o].wOns aHsK thTe1 ;S5K,kCn1o gwennea,s wthaes
dSKetCer1mgienneed, wtoa sbed ethteer msailnt-etdolteorabnet tdheetesramlt-itnoalnert agnetndee ttehrrmouingahn qtugaennteittahtirvoeu gtrhaiqt uloancit i(tQatTivLe) tarnaaitlyloscisi
[(6Q0T].L O)asHnaKlyTs1i;s5[-6d0e]p.eOnsdHenKtT t1ra;5n-sdleopcaetniodne notf tNraan+s ilno craotoiotsn, loefaNf sah+eianthrso,o atns,dl esatefmshse iast ha sk,eayn dmsetcehmansiissma
okfe ysamlt etochlearnainscme tohfrsoaulgthtooluetr athnec egtrhorwotuhg shtoaugetst hoef rgircoew anthd sAtargabeisdoopfsriisc. eAatnHdKATr1a wbidaos pesxisp.rAestsHedK Tm1awinalys
ienx pthrees spehdlomemai ntliyssiunesth aenpdh wloaesm ptriosspuoesseda ntdo wfuansctpioronp ions etdhet orefcuinrccutiloantioinn tohfe Nreac+i. rTcuhlea tfiuonncotifoNn ao+f.
OThsHefKuTn1c;t5io ins corfuOcisaHl, KinTc1lu;5diisngcr tuhcei aplr,ointeccltuiodnin ogf tthhee pnreoxtte gcetinoenraotfiothne seneedxst agnende vriattailo lneasfe bedlasdaens dunvditearl
sleaaltf sbtlraedsse s[6u1n]d. ersaltstress[61].
SSOOSS33 hhaass aann NN--tteerrmmiinnaall mmyyrriissttooyyllaattiioonn ssiiggnnaall ppeeppttiiddee ccrruucciiaall ffoorr SSOOSS33 ffuunnccttiioonn iinn ssaalltt ssttrreessss..
SSOOSS33--lliikkee ccaalclciuiumm-b-bininddininggp rportoeitneisnasr eapreh opsphhosopryhloartyedlatbeydi nbteyr aicnttienrgacwtiinthg SwOSit2h- liSkOeSp2ro-ltiekien kpirnoatseeins,
kanindaspehso, sapnhdo rpyhlaotsiopnhoarpyplaetairosni mapppoertaarnst ifmorptohretaanctt ivfoart iothneo afcktiinvaastieosnb yofc aklicniuasmes-b binyd cinalgcipurmot-ebininsd[i6n2g].
1p4ro-3t-e3inpsr [o6t2e]in. 1s,4r-3e-g3u plartootreyinfsa,c rteogrus,laatroerhyi gfahcltyorcso,n asreer vheigdhilny ecuonkaserryvoetdes iann edukfuanrycotitoens ianndal mfuonscttieovne riny
aalsmpeocstt oefvperlayn atsgpreocwt othf panladntd gevroewlotphm aenndt .d1e4v-e3l-o3ppmroetneti.n 1s4i-n3t-e3r pacrtotweiinths SinOteSr2acatn wdictahn SrOepS2re assndS OcaSn2
raecptirveistsy SuOnSd2e racntoivni-tsyt ruensdsecro nndonit-isotnress,sb cuotnsdoditiiuonms,c baunt rseodduiucemt hceanin rteedruacctei othneb ienttwereaecntio14n- b3e-3twperoente 1in4s-
3a-n3d pSrOotSe2intso aancdti vSaOteS2a ntoS aOcStivpaatteh wana ySOfoSr psaaltthtwolaeyr afnorc esa[6lt3 t]o.lerance [63].
SSOOSS22,, aa llaarrggee ffaammiillyy ooff pprrootteeiinn kkiinnaasseess,, hhaass ttwwoo ssiimmiillaarr kkiinnaasseess iinn yyeeaasstt aanndd mmaammmmaallss:: ssuuccrroossee
nnoonnffeerrmmeennttiinngg 11 ((SSNNFF11) )aanndd AAMMPP-a-catcitviavtaetde dprportoetieni nkikniansaesse (sA(MAPMKPsK). sI)n. pInlanptlsa,n tthse,steh epsreotperinoste ainres
generally referred to as SNF1-related kinases (SnRKs), which play a major role in regulating gene
expression in plant cells [64]. In several plants, members of the SnRK subfamily have been identified
Int.J.Mol.Sci.2018,19,3298 6of16
are generally referred to as SNF1-related kinases (SnRKs), which play a major role in regulating
gene expression in plant cells [64]. In several plants, members of the SnRK subfamily have
been identified [65,66]. The first identified plant SnRK is the rice homolog of SNF1-encoded
protein-serine/threonine kinase (RKIN1), which was found in rye endosperm [67]. According to
its amino acid sequences, and based on the evolutionary relationships, the SnRK family has been
dividedintothreesubfamilies: SnRK1,SnRK2,andSnRK3[68]. TheSnRK1subfamilyhasahighly
conserved(ca.62–64%aminoacididentity)N-terminalcatalyticdomainthathasthreemembers[68,69].
Through gain of function experiments, the SnRK1 subfamily was shown to complement yeast
snf1-defectivemutants. ThissuggestedthattheSnRK1subfamilyisinvolvedinglucosesignaling
and transcription regulation [70]. Compared to the members of the SnRK1 subfamily, the SnRK2
subfamilyhasaconserved(ca. 42–46%aminoacididentity)N-terminalcatalyticdomain,andhas
relatively short C-terminal conserved domains. In rice, the SnRK2 subfamily has 10 members,
OsSAPK1−OsSAPK10[71],andSnRK2subfamilyproteinsthatareessentialtobothosmoticstress
responsesandABAsignaling[72,73].
WhentheCa2+concentrationchanges,plantsuseacalciumeffectorproteintosensethissignal,
andthenmanagetheexternalstimulationbyregulatingtheexpressionoftheplantstressgene.Allthese
effectorproteinshaveanEF-handdomain,definedbyitshelix-loop-helixsecondarystructure,tobind
Ca2+[74].SnRK3sarerareproteinkinasesinplants,calledcalcineurinB-likecalciumsensor-interacting
protein kinases (CIPK or PKS), and SnRK3s interact with one or more members of the family of
SOS3-likecalcium-bindingproteins(SCaBPsorCBLs)throughacommonmotifknownasNAF/FISL
intheN-terminalregulatoryregionofthekinases[48,75].
3.2. Potassium
Potassium (K+) is the most commonly found cation in living plant cells. It is involved in
many aspects of plant growth and development. It can affect all aspects of crop production and
tolerancetovariousabioticstresses[76]. Thus,themaintenanceofK+iontransportersandchannels
across the plasma membrane is essential for proper K+ homeostasis in plants. The large number
of possible CBL−CIPK combinations suggests that the Ca2+−SOS3−SOS2 signaling pathway is
widely used in plants. The maintenance of a high cytosolic K+/Na+ ratio is essential for plant
salt tolerance. During low potassium (K+) stress, which presumably triggers a cytosolic calcium
signal,theCBL1/CBL9−CIPK23moduleactivatestheaffinityK+ transporter(AKT)forK+ uptake
(Figure2)[77].AKT1isoneofthemostimportantK+transportersinArabidopsis.Itmediatescontinuous
growthbyabsorptionofK+byplantrootsthroughvariousexogenousK+concentrationsandpromotes
highaffinityK+uptakeinthelow-K+concentrationrange[78].Theresultsofcipk23,cbl1/cbl9,andakt1
mutants showed similar reduced growth and chlorotic leaves under low K+ conditions [78–80].
Insteadofmutantexperiments,AKT1overexpressed(OE)inArabidopsisdidnotshowanysignificant
improvementingrowthwhentheyweregrowninlowK+conditions,whereasAt/PeCBL1,AtCBL9,
andAtCIPK23OEplantsdemonstratedacomparativetolerancecomparedtocontrolplantsunderthe
sameconditions[81,82]. Inrice,theinwardK+channel,OsAKT1,functionsinK+uptakeinriceroots,
whoseactivityisregulatedbyOsCBL1andOsCIPK23[83]. TheCBL−CIPKnetworkisinvolvedinthe
negativeregulationofAKT1activity. AtCBL10isthoughttobedirectlyboundtoAKT1incompetition
withAtCIPK23,soAtCIPK23stopsbindingandactivatingAKT1[84]. OsAKT1isexpressedinalmost
allricetissuesandorgans,whichcontributesconsiderablytoK+uptakeinawiderangeofexternalK+
concentrations. OsAKT1impactsstomatalconductanceduringosmoticstress,whichisimportantfor
waterstress[76].
AKT2isanotherK+ transporterinvolvedinmovingK+ acrosstheplasmamembrane[85–87].
TheCBL4−CIPK6complexcontrolstheplasmamembranetargetingoftheArabidopsisK+ channel
AKT2(Figure2)[88]. The(de)phosphorylationnetworkregulatesthefunctionalswitchfrominflux
to efflux. In Arabidopsis, protein phosphatase 2C (AtPP2CA) dephosphorylation was found to be
able to repress the ability of AKT2 to move K+ out of the cell [89]. The knockout mutant akt2
Int.J.Mol.Sci.2018,19,3298 7of16
reducedK+ dependenceofthephloemandaffectedsugarloadingintothephloem[87]. Thesingle
knockoutmutantscipk6andcbl4reducedgrowthanddelayedbolting, whichissimilartotheakt2
mutant[85,86,88]. Moreover,CBL2andCBL3mayfunctionintheregulationofK+transportbetween
thevacuoleandcytoplasm[90].
The H+-ATPases are important components in the initial sensing in plant responses to
K+ deficiency [91]. The plant K+ transporters are mainly derived from several gene families,
including KUP/HAK/KT, HKT, NHX, and CHX [92]. OsHAK1, OsHAK2 and OsHAK5, as K+
transporters,playimportantrolesinK+acquisitionanddistributioninrice[93]. Theover-expression
of OsHAK1 significantly improved salinity tolerance and drought tolerance [93–95]. The rice
OsHAK2proteinissensitivetoextracellularNa+ andtransportsNa+ moreeffectivelythanK+ [96].
Theover-expressionofOsHAK5inriceplantsincreasedtheshoot[K+]/[Na+]ratioandenhancedthe
salttoleranceoftheplant. OsHAK5playsanimportantroleintheprocessofobtainingpotassiumfrom
rootsthatarefacedwithlowexogenouspotassiumandupwardmigrationofpotassium,transporting
potassiumfromrootstoabovegroundpartsinK-deficientriceplants[97].
3.3. Nitrate
Nitrogen is important for crop production and plant growth, and is highly regulated and
coordinated with other transport and metabolic pathways [98,99]. Plants mainly uptake NO − as
3
thenitrogensource[100]. Therearetwosystemsfornitrateuptake: thehigh-affinitysystemandthe
low-affinitysystemfornitrateuptake[101,102]. Thehigh-affinitysystemisinducedinnitratedeficit,
whilethelow-affinitysystemisusedfortheprimarynitrateuptakeandresponseunderasufficient
situation[101,102]. Inplants,therearethreenitratetransporterfamilies: nitratetransporter1(NRT1),
nitratetransporter2(NRT2)andchloridechannel(CLC).Thefirstidentifiednitratetransporterwas
NRT1,orchlorateresistant1(CHL1),whichisadose-dependentmastercontrollerofmultiplesignaling
mechanisms,capableofrespondingtoawiderangeofsoilnitratelevels[103,104].Thereare53AtNRT1,
7AtNRT2and7AtCLCthathavebeenidentifiedinArabidopsis[103,105,106]. Amongthem,AtNRT2.1
andAtNRT2.2areengagedinhigh-affinityuptake.AtNRT1.1isinvolvedinbothhigh-andlow-affinity
uptakeofNO −,whereasAtNRT1.2worksinlow-affinityuptake[107,108]. Inrice,upto250-fold
3
increasesintheinductionofgeneexpressionafternitrateresupplywereobservedforthehigh-affinity
transportersOsNRT2.1andOsNRT2.2[109].
Whenfacedwithalackofnitrate,AtCBL19-AtCIPK23compoundsareresponsibleforNRT1.1
phosphorylation,whichimprovesthehighbindingaffinityandtransportcapacity,inordertoincrease
theabsorptionofnitrate[110]. AtCIPK8mayparticipateinresponsetohighconcentrationsofnitrate
byperceivingandactivatingthelowaffinitynitratereaction[111]. Experimentsonthecipk8mutant
showedthatAtCIPK8participatesinthelong-termprocessofnitrate-regulatingrootgrowth,andthe
responsetoprimarynitrateproducedapositiveeffect[111].
3.4. Phosphorus
Phosphorus is an essential nutrient for plant growth and development that accounts for
about 0.2% of a plant’s dry weight [112]. It is also a constituent of nucleic acids and membrane
phospholipids.HighefficiencyPnutritioninplantsisrelatedtomanyfactors,suchasrootmorphology
and root exudates [113]. Plants can absorb Pi from the soil as an inorganic orthophosphate ion,
but the availability is strictly limited by reactions of inorganic and organic phosphates with soil
constituents[114].
The CBL−CIPK system is also involved during the response to low Pi in Brassica napus [115].
Yeast two-hybrid analysis revealed that BnCIPK6 is able to interact with Arabidopsis CBL1, CBL2,
CBL3andCBL9. BnCBL1andBnCIPK6wereupregulatedinPideficiencytreatmentandbothproteins
interactedwitheachotherinyeasttwo-hybridscreensandasplit-yellowfluorescentprotein(YFP)
system. ThismeansthatBnCBL1andBnCIPK6regulatetheprocessesinvolvedintheplant’sresponse
Int.J.Mol.Sci.2018,19,3298 8of16
toPideficiency[115]. OverexpressionofeitherBnCBL1orBnCIPK6canenhancegrowthandbiomass
productionunderlowPistressinArabidopsis[115].
3.5. Magnesium
Although plants rely on a sufficient supply of Mg2+ for normal growth and development,
excessiveMg2+accumulationoftencausestoxicitytoplantcells[116]. TwoCBLproteins,CBL2and
CBL3, act as key regulators for plant growth under high-Mg conditions [117]. The cbl2 cbl3
double-mutant plants retained a lower Mg content than wild-type plants under either normal
or high-Mg conditions, suggesting that CBL2 and CBL3 may be required for vacuolar Mg2+
sequestration[117]. FourCIPKs;CIPK3,CIPK9,CIPK23andCIPK26,actasfunctionallyoverlapping
componentsdownstreamofCBL2/3inthesignalingpathwaythatfacilitatesMg2+homeostasis[117].
ThisMg2+partitioningprocessinthevacuole,controlledbytheCBL−CIPKpathway,mayrepresenta
generalmechanismunderlyingdetoxification[118].
3.6. Calcium
Calciumactsasacriticalmessengerintheadaptationanddevelopmentalprocessesofplants.
Generally, Ca2+ transmits the stress signal through a downstream pathway by binding to protein
sensors called CBL, which interact with CIPKs, a specific group of protein kinases [75]. In plants,
theproteinCBLfamilyrepresentsauniquegroupofcalciumsensorsandhelpstodecodecalcium
transients by specifically interacting with, and regulating, a family of protein kinases (CIPKs).
Theuniquefeatureofthecbl10mutant, despitebeingmoresensitivetosalt, isthatitaccumulates
significantlylesssaltthanthewild-type[119].DifferentCBLsandtheirtargetkinaseshavebeenshown
tofunctioninstressresponses. CBL4(SOS3),anditsinteractingkinaseCIPK24(SOS2),togetherwith
SOS1,constituteapathwaythatmayfunctioninNa+exclusionfromthecytoplasm[2].
Ca2+-dependent pathways always play critical roles in salt stress responses [120]. Ca2+ is the
secondmostimportantmessengerintermsofabioticstressresponses,suchassalinitytolerance[121].
InthecaseofNa+ionexclusionfromthecytoplasm,Ca2+sensorproteinCBL4(SOS3)interactswith
the protein kinase CIPK24 (SOS2) and another Na+/H+ exchanger pathway (SOS1) at the plasma
membrane[122]. TheCa2+-stresssignalingsystemiscomplex;theCa2+signalingprocessisactivated
withthepresenceofaCa2+sensorandtheirtargetproteins.
4. Discussion
Plantsareexposedtovariousadversestressconditionsduringtheirgrowthanddevelopment
processes. The stress sensor identification remains a vital part of abiotic stress research.
Through mutant analysis and the CRISPR/CAS9 approach, an increasing number of genes that
arerelativetostresssensorswillbestudieddeeply.
ThediscoveryoftheCBLandCIPKnetworkrepresentsanexampleofasignificantlydiverged
Ca2+-decodingmodelforstresssensingthatisdistinctinplants. Itisakeysignalingsysteminvarious
stress signaling pathways in plants [123]. Different CBL proteins have been reported to interact
withdifferentCIPKproteins,andthetypesofthisinteractiondeterminethenetworkoutcome[122].
To date, CBL and CIPK members have been comprehensively analyzed in many species (Table 2).
Inrice,therearemorethan30OsCIPKs,mostofwhichrespondtoatleastonestressfactoramongsalt,
droughtandotherabioticstresses. OsCIPK3,OsCIPK12andOsCIPK15functionaspositiveregulators
ofcold,droughtandsaltstresstolerance,respectively[124].OsCIPK3wasshowntonegativelyregulate
saltstresstoleranceinrice[125]. Duringtheseedgerminationandseedlingstage,OsCIPK31/OsCK1
wasassociatedwithabioticstressresponses,whichcanleadtothedifferentialexpressionofvarious
stress-responsivegenes[126]. OsCIPK14andOsCIPK15playanimportantroleinthedefensesignaling
pathwaytriggeredbymicrobesinculturedricecells[127].
Int.J.Mol.Sci.2018,19,3298 9of16
Table2. CalcineurinB-likeproteins(CBL)andCBL-interactingproteinkinase(CIPK)membersin
differentspecies.
Species CBLs CIPKs Reference
Arabidopsisthaliana 10 26 [120,128,129]
O.sativa 10 31 [128]
TriticumaestivumL. 7 20 [130]
Z.mays 8 43 [131]
Populustrichocarpa 10 27 [132,133]
B.napusL. 7 23 [129]
S.melongenaL. 5 15 [134]
Malusdomestica - 34 [135]
Many genes for putative CIPK proteins were found in the Arabidopsis genome. In previous
studies, the exchange of sodium (Na+), potassium (K+) and nitrate (NO3−) ion transportation
across the plasma membrane and tonoplast in Arabidopsis was demonstrated to be regulated by
theCBL−CIPKpathway[133]. cDNAcloningandsequencinghavebeenconfirmedinatleast25CIPK
genes. The interaction of CIPK24/SOS2 and CBL4/SOS3 was identified by a genetic screen that
highlightedaroleinsalttoleranceinArabidopsis[136]. Inthisconnection,studieshaveestablishedthat
CBL4/SOS3−CIPK24/SOS2directlyregulatesthedownstreamcomponentSOS1,aputativeNa+/H+
antiporterthatinteractswiththeNa+iondetoxificationprocess[137],aswellaslow-K-tolerantmutants
CIPK23, a critical K-nutrition determinant in Arabidopsis, was identified in a forward genetic
screen[78]. CBL10,inasalttolerantpathway,wasmainlyexpressedandfunctionedintheshootsand
leaves,unlikeCBL4,anothersalttolerantpathway,thatonlyworkedintheroots. TheCBL4(SOS)
pathwayinteractedwithCIPK24(SOS2)proteinkinasetoexportsaltfromtheplasmamembrane[119].
ThesestudiessupporttheideathatCBL−CIPKnetworksplayimportantrolesinabioticstresssensing
andregulatingionicbalances.
Duringevolution,plantsadoptdifferentmorphologicalandphysiologicalchangesinresponse
todifferentabioticstresses. ResearchhasrevealedthatCBL−CIPKnetworksareintegralinenabling
plants to respond to abiotic stress and coordinate essential defense mechanisms [133]. By better
understandingtheCBL−CIPKsstress-sensingpathway,theionicstressresponsemechanismofplants
shouldbeeasilyrevealed. Previously,mostoftheresearchinCBLandCIPKfocusedonidentifying
the interactions between CBLs and CIPKs, the location of their interaction and the phenotypic
analysis of CBL or CIPK mutants exposed to different abiotic stresses [80]. Further experiments
haveextendedtheanalysisofCBL−CIPKinteractionstotheentirefamilyofCBLsandalargefraction
of the CIPK family in an effort to determine functional pairs of CBLs and CIPKs [120]. However,
determiningthedownstreamtargetsisessentialtogainingacompleteunderstandingoftheCIPKand
CBLsignalingnetwork.
Acknowledgments: This work was supported by National Science-technology Support Projects (2015BAD
01B02-02)andNationalNaturalScienceFoundationofChina(31461143014and31601277).
ConflictsofInterest:Theauthorsdeclarenoconflictsofinterest.
References
1. Manik,S.M.;Shi,S.;Mao,J.;Dong,L.;Su,Y.;Wang,Q.;Liu,H.ThecalciumsensorCBL-CIPKisinvolvedin
plant’sresponsetoabioticstresses.Int.J.Genom.2015,2015,493191.[CrossRef][PubMed]
2. Zhu,J.K.Abioticstresssignalingandresponsesinplants.Cell2016,167,313–324.[CrossRef][PubMed]
3. Wani,S.H.;Kumar,V.;Shriram,V.;Sah,S.K.Phytohormonesandtheirmetabolicengineeringforabiotic
stresstoleranceincropplants.Crop.J.2016,4,162–176.[CrossRef]
4. Srivastava,A.K.;Penna,S.;Nguyen,D.V.;Tran,L.S.Multifacetedrolesofaquaporinsasmolecularconduits
inplantresponsestoabioticstresses.Crit.Rev.Biotechnol.2016,36,389–398.[CrossRef][PubMed]
Int.J.Mol.Sci.2018,19,3298 10of16
5. Latif,F.;Ullah,F.;Mehmood,S.;Khattak,A.;Khan,A.U.;Khan,S.;Husain,I.Effectsofsalicylicacidon
growthandaccumulationofPhenolicsinZeamaysL.underdroughtstress.ActaAgric.Scand.Sect.BSoil
PlantSci.2015,66,325–332.
6. Gorji,A.H.;Hajianfar,R.;Rostamforody,B.;Production,P.Effectsofcoldandotherabioticstressonplants.
Int.J.Agron.2013,3597–3604.
7. Guo,X.;Liu,D.;Chong,K.Coldsignalinginplants: Insightsintomechanismsandregulation. J.Integr.
PlantBiol.2018,60,745–756.[CrossRef][PubMed]
8. Ma,Y.;Dai,X.;Xu,Y.;Luo,W.;Zheng,X.;Zeng,D.;Pan,Y.;Lin,X.;Liu,H.;Zhang,D.;etal.COLD1confers
chillingtoleranceinrice.Cell2015,162,1209–1221.[CrossRef][PubMed]
9. Li,B.;Gao,K.;Ren,H.;Tang,W.Molecularmechanismsgoverningplantresponsestohightemperatures.
J.Integr.PlantBiol.2018,60,757–779.[CrossRef][PubMed]
10. Jung,J.H.;Domijan,M.;Klose,C.;Biswas,S.;Ezer,D.;Gao,M.;Khattak,A.K.;Box,M.S.;Charoensawan,V.;
Cortijo,S.PhytochromesfunctionasthermosensorsinArabidopsis. Science2016,354,886–889. [CrossRef]
[PubMed]
11. Gangappa,S.N.;Berriri,S.;Kumar,S.V.PIF4coordinatesthermosensorygrowthandimmunityinArabidopsis.
Curr.Biol.2017,27,243–249.[CrossRef][PubMed]
12. Wei,Z.; Yuan,T.; Tarkowská,D.; Kim,J.; Nam,H.G.; Novák,O.; He,K.; Gou,X.; Li,J.Brassinosteroid
biosynthesisismodulatedviaatranscriptionfactorcascadeofCOG1,PIF4,andPIF5.PlantPhysiol.2017,
174,1260–1273.[CrossRef][PubMed]
13. Kumar,M.Cropplantsandabioticstresses.J.Biomol.Res.Ther.2014,03,e125.[CrossRef]
14. Kumar,K.;Kumar,M.;Kim,S.R.;Ryu,H.;Cho,Y.G.Insightsintogenomicsofsaltstressresponseinrice.
Rice2013,6,1–15.[CrossRef][PubMed]
15. Kumar,M.;Choi,J.;An,G.;Kim,S.R.EctopicexpressionofOSSTA2enhancessaltstresstoleranceinrice.
Front.PlantSci.2017,8,316.[CrossRef][PubMed]
16. Fu,R.;Zhang,M.;Zhao,Y.;He,X.;Ding,C.;Wang,S.;Feng,Y.;Song,X.;Li,P.;Wang,B.Identificationofsalt
tolerance-relatedmicroRNAsandtheirtargetsinmaize(ZeamaysL.)usinghigh-throughputsequencingand
degradomeanalysis.Front.PlantSci.2017,8,864.[CrossRef][PubMed]
17. Xie,R.;Zhang,J.;Ma,Y.;Pan,X.;Dong,C.;Pang,S.;He,S.;Deng,L.;Yi,S.;Zheng,Y.;etal. Combined
analysisofmRNAandmiRNAidentifiesdehydrationandsalinityresponsivekeymolecularplayersin
citrusroots.Sci.Rep.2017,7,42094.[CrossRef][PubMed]
18. Liu,Q.;Yan,S.;Yang,T.;Zhang,S.;Chen,Y.Q.;Liu,B.SmallRNAsinregulatingtemperaturestressresponse
inplants.J.Integr.PlantBiol.2017,59,774–791.[CrossRef][PubMed]
19. Huang,J.;Yang,M.;Zhang,X.ThefunctionofsmallRNAsinplantbioticstressresponse.J.Integr.PlantBiol.
2016,58,312–327.[CrossRef][PubMed]
20. DeZelicourt,A.;Colcombet,J.;Hirt,H.TheroleofMAPKmodulesandABAduringabioticstresssignaling.
TrendsPlant.Sci.2016,21,677–685.[CrossRef][PubMed]
21. Zhu,J.K.Saltanddroughtstresssignaltransductioninplants. Annu. Rev. Plant. Biol. 2002,53,247–273.
[CrossRef][PubMed]
22. Tenhaken,R.Cellwallremodelingunderabioticstress.Front.PlantSci.2014,5,771.[CrossRef][PubMed]
23. Liu, J.X.; Howell, S.H. Managing the protein folding demands in the endoplasmic reticulum of plants.
NewPhytol.2016,211,418–428.[CrossRef][PubMed]
24. Banerjee,A.;Roychoudhury,A.Abscisic-acid-dependentbasicleucinezipper(BZIP)transcriptionfactorsin
plantabioticstress.Protoplasma2017,254,3–16.[CrossRef][PubMed]
25. Jin,J.;Tian,F.;Yang,D.C.;Meng,Y.Q.;Kong,L.;Luo,J.;Gao,G.PlantTFDB4.0: TowardacentralHUB
fortranscriptionfactorsandregulatoryinteractionsinplants. NucleicAcidsRes. 2017,45,D1040–D1045.
[CrossRef][PubMed]
26. Mignolet-Spruyt,L.;Xu,E.;Idanheimo,N.;Hoeberichts,F.A.;Muhlenbock,P.;Brosche,M.;vanBreusegem,F.;
Kangasjarvi,J.Spreadingthenews: Subcellularandorganellarreactiveoxygenspeciesproductionand
signalling.J.Exp.Bot.2016,67,3831–3844.[CrossRef][PubMed]
27. Bobik,K.;Burch-Smith,T.M.Chloroplastsignalingwithin,betweenandbeyondcells.Front.PlantSci.2015,
6,781.[CrossRef][PubMed]